1. Field of the Invention
The present invention relates to the design of semiconductor light-emitting devices. More specifically, the present invention relates to a technique for fabricating a low-resistivity ohmic contact to a p-type III-V nitride semiconductor material and a method for fabricating semiconductor light-emitting devices.
2. Related Art
Group III-V nitride compounds (e.g., GaN, InN, and AlN) and alloy-compounds (e.g., AlGaN, InGaN, and AlGAInN) have been demonstrated to generate efficient luminescence at the blue-green visible spectrum. This fact has been the driving force for their recent technological development in light-emitting diodes (LEDs) and laser diodes. For example, high-brightness LEDs using group III-V nitride materials have transformed the market for color displays and opened the door to many field applications (e.g., in traffic lights and flat-panel white light sources). In addition, UV laser diodes using group III-V nitride materials have also been widely used in scientific instrumentation, laboratories, and commercial products.
One important factor that affects the fabrication of light-emitting devices is the nature of a P-N junction. When p-type and n-type materials are placed in contact with each other, the junction behaves differently from either type of material alone. More specifically, when a forward-bias voltage is applied across the P-N junction, the carriers, namely holes from the p-type layer and electrons from the n-type layer, recombine in the P-N junction region and release energy in the form of photons. In addition, an active region can be formed by a multi-quantum-well (MQW) structure between the p-type layer and the n-type layer. This MQW structure produces a higher carrier density by restricting carriers between quantum barriers and hence increases the carrier recombination rate. The faster the carriers recombine, the more efficient a light-emitting device becomes.
Techniques for epitaxially growing an LED structure with group III-V nitride materials include, but are not limited to, metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and Hydride Vapor Phase Epitaxy (HVPE). Substrate materials used for epitaxial growth include sapphire (AL2O3), silicon, and silicon carbide (SiC). When Si and magnesium (Mg) are used as the donor and acceptor dopants respectively for fabricating group III-V nitride materials, it is relatively easy to obtain high carrier density in n-type nitride materials. However, high carrier density is more difficult to obtain for p-type nitride materials.